\section{Introduction}
\label{intro}
The term ontology originates from the Greek words for being and science. In philosophy it refers to "the study of the nature of being, existence or reality as such, as well as the basic categories of being and their relations"\footnote{\url{http://en.wikipedia.org/wiki/Ontology}}. In the context of computer science, an ontology is a "formal representation of knowledge as a set of concepts within a domain, and the relationships between those concepts"\footnote{\url{http://en.wikipedia.org/wiki/Ontology_(information_science)}}. 

Tom Gruber coined the term in \cite{t_gruber_ontology} as a ``formal, explicit specification of a shared conceptualization''. In research as well as business applications it is very important to have sophisticated tools to generate (complex) ontologies. Especially when multiple domain experts and technicians work on the same shared concepts a collaborative modeling approach is needed in order to guarantee good results. Many different ontology editors have been developed since the 1980's and this paper presents some of these tools that support ontology engineering. These tools contain open source (OSS\footnote{Open source software}), commercial as well as collaborative solutions. A section about related work and conclusions complete the paper.